This invention relates to a method and apparatus for assessing the electrical stability of the heart. Electrical stability is characteristic of proper functioning of the heart. Electrical instabilities, characterize, or indicate, a potential for abnormal conditions, including such serious conditions as ventricular fibrillation, which may result in sudden cardiac death.
It is known that electrocardiographic (ECG) analysis may be used to identify individuals having increased risk of myocardial electrical instability. The electrocardiograms of such individuals evidence changes in the morphology or shape of the electrocardiographic signal complex. Changes in the amplitude or temporal distribution of the T-wave portion of the electrocardiographic signal complex can be used for analysis purposes. These changes in the morphology of the electrocardiographic signal may appear in every other beat of the heart so that the electrocardiographic complex will have, for example, a T-wave of one configuration in a first beat, a T-wave of a different configuration in a second beat, a T-wave configuration resembling that of the first beat in the third beat, a T-wave configuration resembling that of the second beat in the fourth beat, and so on. The alternating signal produced by this phenomena is often termed “altemans.” Because the alternation occurs on an every other beat basis, the frequency of the alternating signal is 0.5 alternations/beat or 0.5 cycles per beat (CPB).
One non-invasive technique for assessing myocardial electrical stability of a given individual involves analysis of the alternation from beat-to-beat in the morphology of the electrocardiogram (ECG) complex. Such a technique is described in U.S. Pat. Nos. 4,802,491, 5,570,696, and 5,713,367. The prior art comprehends the relationship of the fluctuations in the T-wave morphology of the ECG complex with susceptibility to ventricular fibrillation. In the prior art, the presence of alternation is detected using spectral analysis of the beat-to-beat T-wave amplitude in a single ECG lead, and measuring the excess in the unique spectral line at a frequency of 0.5 alternations/beat. The reference for T-wave amplitude measurement is the onset of QRS portion of the electrocardiographic signal. Emphasis is made on the small amplitudes of alternation to be found. Given the evidence of the small nature to the effect to be detected, it is clear that successful detection of alternation will require careful optimization of variable to be measured.